Upon antigen engagement and proper co-stimulation, naïve lymphocytes exit quiescence and undergo clonal expansion and differentiate into functional effector cells, after which they either die through apoptosis or survive as memory cells. Lymphocytes at different activation stages exhibit distinct metabolic signatures. Emerging evidence highlights a central role for the mechanistic target of rapamycin (mTOR) in bridging immune signals and metabolic cues to direct lymphocyte proliferation, differentiation and survival. Here we review recent advances in understanding the functional significance and signal transduction of mTOR in T cell biology, and the interplay between mTOR signaling and metabolic programs.